The overall objective of this project is to characterize a progesterone-induced factor which controls nuclear estrogen receptor levels. This substance is called estrogen receptor-regulatory factor (Re-RF). Experiments have been designed using hamster uterus to: 1) establish assay conditions for measurment of Re-RF in cytosol and nucleus, 2) purify and study the physicochemical properties of Re-RF, 3) study the binding of Re-RF to estrogen receptor and nuclear acceptor sites, 4) prepare an antibody to Re-RF, and 5) further study the hormonal regulation of this factor.